Surface phase diagram of (2×4) and (4×2) reconstructions of GaAs(001)

Total-energy calculations for a series of $(2\mathrm{}\times 4)$ and $(4\mathrm{}\times 2)$ reconstructed GaAs(001) surfaces not included in previous theoretical studies are presented. A $(2\mathrm{}\times 4)$ surface model containing single anion dimers in the first and third atomic layers is predicted for a balanced surface stoichiometry. It is more stable than the two-As-dimer $\alpha$ structure assumed previously, due to its lower electrostatic energy. Our results for the $(4\mathrm{}\times 2)$ reconstructed surface confirm the two-Ga-dimer $\beta 2$ structure suggested by Biegelsen and co-workers. Nearly degenerate in energy, however, are mixed Ga-As dimers adsorbed on a Ga-terminated substrate.